During the last decades, different types of batteries have been developed to respond to size, weight, and capacity requirements depending on the nature of the electronic devices. For example, lithium-ion batteries are particularly well adapted to portable electronic equipment in terms of energy density and of time stability (charge/discharge cycles).
Generally, a lithium-ion battery is an assembly of a positive electrode (cathode), comprising a lithium-based material, and of a negative electrode (anode) generally made from carbon (graphite, for example). Its operation is ensured by the reversible exchange of Li+ ions between the cathode and the anode, the electrodes being separated by an electrolyte based on lithium salt.
In the development of lithium-ion batteries, many positive electrode materials have been tested, and particularly LiMPO4 phosphates (M=Mn, Fe, or Co). Such materials are advantageous and arouse much interest due to the security that they provide and to their low cost. However, their theoretical specific capacity remains limited to 170 mAh/g for LiFePO4.
To obtain batteries having higher specific capacities, other materials have been envisaged, particularly LiMBO3 borates, with M=Mn, Fe, or Co. Such materials have the advantage of having a maximum theoretical capacity (220 mAh/g) greater than that of LiMPO4 phosphates, while being as attractive in terms of security.
However, the redox potentials of couples Fe2+/Fe3+ and Mn2+/Mn3+ are relatively low, which results in limiting the energy density of the LiFeBO3 and LiMnBO3 compounds.
The LiCoBO3 compound enables to improve the energy density, given that the Co2+/Co3+ redox couple of cobalt has a higher potential than that of couples Fe2+/Fe3+ and Mn2+/Mn3+. However, the disadvantage of the LiCoBO3 compound with respect to LiFeBO3 and LiMnBO3 compounds is its rather low experimental reversible capacity.
There thus is a need to improve the properties of lithium-borate materials, by developing a material having the following properties:                a higher average potential;        a good specific mass capacity; and        a good stability over time regarding charge/discharge cycles.        
The present invention relates to lithium-borate compounds, having properties enabling to solve this problem. They comprise at the same time the two transition metals, manganese and cobalt.
Further, the Applicant has observed that prior art methods do not enable to prepare such compounds comprising at the same time lithium manganese and cobalt borates. Such methods do not result in the forming of a single phase containing the manganese and the cobalt.
To overcome this technical issue, the Applicant has developed a novel method in a plurality of steps, which takes into account possible incompatibilities relative to the respective reactivities of the cobalt and manganese compounds.